1. Field of the Invention
The present invention relates to an ionization chamber, and in particular to a liquid chromatograph mass spectrometer having an ionization chamber for ionizing a liquid sample fed from a liquid chromatograph unit and a mass spectrometer unit into which ions are introduced from the ionization chamber.
2. Description of Related Art
Liquid chromatograph mass spectrometers (LC/MS) are formed of a liquid chromatograph unit (LC unit) for eluting a liquid sample so that the liquid sample is separated into respective components, an ionization chamber (interface unit) for ionizing the sample components that have been eluted from the LC unit and a mass spectrometer unit (MS unit) for detecting the ions that have been introduced from the ionization chamber. In such ionization chambers, various ionization techniques are used in order to ionize a liquid sample, and atmospheric pressure ionization methods such as atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) are widely used.
In accordance with APCI specifically, the end of a nozzle connected to the terminal of the column in the LC unit is directed toward the inside of the ionization chamber, and at the same time a needle electrode is provided in front of the end of the nozzle. Thus, droplets of the sample that has been atomized through the application of heat in the nozzle are ionized through a chemical reaction with carrier gas ions (buffer ions) that have been generated by means of corona discharge from the needle electrode. In accordance with ESI, the end of a nozzle connected to the terminal of the column in the LC unit is directed toward the inside of the ionization chamber, and at the same time a high voltage of approximately 5 kV is applied to the end portion of the nozzle so that an intense non-uniform electric field is generated. Thus, the liquid sample undergoes charge separation in the electric field so as to be torn off for atomization by means of coulomb attraction. As a result, the solvent in the droplets of the sample evaporates after coming into contact with the surrounding air so that gas ions are generated.
As described above, a liquid sample is ionized in such a state that the sample is placed under pressure that is close to atmospheric pressure in accordance with APCI or ESI. Therefore, a structure is adopted such that middle chambers or the like are provided between the ionization chamber in a high pressure state (that is to say, a state that is close to atmospheric pressure) and the MS unit in a very low pressure state (that is to say a highly vacuumed state) so that the degree of vacuum is increased incrementally in order to secure the difference in the pressure between the ionization chamber and the MS unit (see Patent Document 1).
FIG. 7 is a schematic diagram showing the structure of an example of a liquid chromatograph mass spectrometer in accordance with an ESI method. Here, a certain direction that is horizontal relative to the ground is the X direction, the direction that is horizontal relative to the ground and perpendicular to the X direction is the Y direction, and the direction that is perpendicular to the X direction and the Y direction is the Z direction.
A liquid chromatograph mass spectrometer 101 is provided with a liquid chromatograph unit (LC unit) 60, an ionization chamber 200 and a mass spectrometer unit 50. In addition, a first middle chamber 12 that is adjacent to the ionization chamber 200, a second middle chamber 13 that is adjacent to the first middle chamber 12 and a mass spectrometer chamber (MS unit) 14 that is adjacent to the second middle chamber 13 are provided sequentially, with partitions in between them, in the mass spectrometer unit 50.
The liquid sample that has been separated into the respective components in the LC unit 60 is supplied through a flow path 155. In addition, a nebulizer gas (nitrogen gas) is supplied through a flow path 156. As a result, the liquid sample and the nebulizer gas are introduced into a spray 15 for atomization.
FIGS. 8A and 8B are side diagrams showing the spray. FIG. 8B is a cross-section diagram showing an enlargement of A in FIG. 8A. The spray (atomization means) 15 has a probe main body 151 and a nozzle 152 for atomizing a liquid sample.
The nozzle 152 has a double-pipe structure that is formed so as to protrude downward from the bottom of the probe main body 151. The liquid sample that is supplied through the flow path 155 is ejected from the inside of the internal circular pipe (having an outer diameter of 0.27 mm, for example) 152b. Meanwhile, the nitrogen gas supplied through the flow path 156 is ejected between the internal circular pipe 152b and the external circular pipe (having an inner diameter of 0.37 mm, for example) 152a. As a result, the ejected liquid sample is sprayed in an atomized state due to the effects of impact with the nebulizer gas that is ejected from the space surrounding the internal circular pipe 152b. In addition, wires (not shown) are connected to the end of the external circular pipe 152a so that a high voltage of approximately 5 kV is applied from the power supply (not shown) in order to achieve ionization.
In addition, the nozzle 152 can move approximately parallel to the probe main body 151 within a predetermined range in the XY plane perpendicular to the Z direction by means of a position-adjusting knob (not shown), and thus the position of nozzle 152 can be fixed using a position-fixing knob after the position has been adjusted appropriately. Furthermore, the nozzle 152 can be inserted and extracted in the Z direction relative to the probe main body 151 (adjustment of the extent of protrusion) and the position of the nozzle 152 can be fixed by means of a nut or the like after the position has been adjusted appropriately.
While in FIGS. 8A and 8B the spray 15 is for ESI, in general the spray 15 is removable from the ionization chamber 200. In the case where an APCI method is used, the spray 15 is removed and instead a spray for APCI, where the needle electrode for charging forms a unit, is attached to the ionization chamber 200.
The ionization chamber 200 is provided with a sub-chamber 210 in a rectangular parallelepiped form of 13 cm×13 cm×12 cm. The sub-chamber 210 has an upper surface, a front surface, a right-side surface, a rear surface (partition 26), a left-side surface and a lower surface. Thus, an internal space surrounded by six surfaces—upper, lower, left, right, front and rear—is formed in the ionization chamber 200.
In addition, a circular opening (not shown) that runs through in the upward and downward directions (Z direction) is created in the upper surface so that a spray 15 can be attached to the opening from the top. Furthermore, a drain 211 is formed on the lower surface so that the unnecessary liquid sample can be discharged to the outside through the drain 211.
Moreover, the partition 26 is provided so as to separate the inside of the sub-chamber 210 from the inside of the first middle chamber 12. A heater block 20 in a rectangular parallelepiped form into which a temperature adjusting mechanism (not shown) is incorporated is fixed in the center portion of the partition 26. FIG. 9 is a diagram showing the structure of the heater block 20 that is provided on the partition 26 in the ionization chamber 200 in FIG. 7.
One desolvation pipe (ion introducing pipe) 119 of which the entrance is provided inside the sub-chamber 210 and of which the exit is provided inside the first middle chamber 12 is formed in the heater block 20. The desolvation pipe 119 is in a circular pipe form having the center axis in the X direction (having an outer diameter of 1.6 mm and an inner diameter of 0.5 mm, for example). As a result, the entrance of the desolvation pipe 119 is pointed in a direction (X direction) that forms approximately a right angle relative to the direction in which the sample is sprayed from the nozzle 152 (Z direction), and a gigantic droplet of the sample that has been sprayed is thus prevented from directly flying into the desolvation pipe 119.
In addition, six dry gas pipes 218 of which the exits are provided inside the sub-chamber 210 are formed in the heater block 20. Each dry gas pipe 218 is in a circular pipe form (having a diameter of 0.5 mm, for example) of which the center axis is in the X direction. The six dry gas pipes 218 are arranged at equal intervals in a circle with the desolvation pipe 119 at the center.
Thus, the partition 26 of the sub-chamber 210 accelerates desolvation and ionization through the effects of the application of heat and of impact when ions and microscopic droplets of the sample that have been sprayed from the nozzle 152 pass through the inside of the desolvation pipe 119.
A first ion lens 21 is provided inside the first middle chamber 12 and an exhaust vent 31 for discharging air by an oil-sealed rotary pump (RP) so as to create a vacuum of approximately 102 Pa is provided in the lower surface of the first middle chamber 12. A skimmer 22 having an orifice is formed in the partition between the first middle chamber 12 and the second middle chamber 13, and the inside of the first middle chamber 12 and the inside of the second middle chamber 13 are connected through this orifice.
An octapole 23 and a focus lens 24 are provided inside the second middle chamber 13, and an exhaust vent 32 for discharging air by means of a turbo molecular pump (TMP) so as to create a vacuum of approximately 10−1 Pa to 10−2 Pa is provided in the lower surface of the second middle chamber 13. An entrance lens 25 having an orifice is provided in the partition between the second middle chamber 13 and the mass spectrometer chamber 14, and the inside of the second middle chamber 13 and the inside of the mass spectrometer chamber 14 are connected through this orifice.
A first quadrupole 16, a second quadrupole 17 and a detector 18 are provided inside the mass spectrometer chamber 14, and an exhaust vent 33 for discharging air by means of a turbo molecular pump (TMP) so as to create a vacuum of approximately 10−3 Pa to 10−4 Pa is provided in the lower surface of the mass spectrometer chamber 14.
In the thus formed liquid chromatograph mass spectrometer 101, the ions that have been generated in the ionization chamber 200 pass through the desolvation pipe 119, the first ion lens 21 located within the first middle chamber 12, the skimmer 22, the octapole 23 and the focus lens 24 located within the second middle chamber 13 and the entrance lens 25 in this order so as to be fed into the mass spectrometer chamber 14. In this chamber 14 unnecessary ions are discharged by means of the quadrupoles 16 and 17 and only the specific ions that have reached the detector 18 can be detected.